Continuous Wave (CW) Coherent Radar uses frequency conversion to compare the phase of a transmitted signal with the reflection of that signal from a moving target. The phase of the wave reflected from the target changes as a function of the changing distance to that target. If the target velocity remains constant, the phase of the reflected signal changes at a constant rate. A constant rate of phase change corresponds to a constant frequency. Thus the returning reflected signal appears at a frequency offset from the transmitted signal that is proportional to the relative velocity between the transmitter and the target.
Comparing the transmitted signal and the received signal with a frequency downconverter delivers the difference frequency between the transmitted and received signals at the converter output. Practical implementation of a radar seeks to optimize the cost and size of the equipment required to compare the phase (or frequency) of the transmitted and received signals, while obtaining the greatest detection range to target possible for that cost and size.
Many conventional portable radar guns use a Gunn diode driving a cavity oscillator with an integral diode peak detector which functions as a frequency downconverter or mixer, using either one or multiple detector diodes. The cavity oscillator/mixer is coupled to a horn antenna used to transmit the incident signal and to receive the reflected signal. The cavity drives the diode detector with a local oscillator (“LO”) signal from the transmitter and couples the received RF signal to the same diode. The diode detector mixes the RF and LO signals, creating an IF signal at their difference frequency. The diode detector typically matches to a relatively high impedance, often hundreds or even thousands of ohms, and conversion loss can approach 0 dB. Matching to LO and RF signals is accomplished by moving the diode location within the cavity to optimize the coupling for optimal system performance.
The detector diode also rectifies the LO power in the cavity, and any variations in the amplitude due to either coherent amplitude modulation (“AM”) or to AM noise will show up at the IF output. Because of this problem, designers typically use Gunn diode oscillators adjusted to the point of minimal conversion of diode bias supply voltage input to amplitude variation. This minimizes the AM noise on the LO and thus also minimizes the detected LO AM noise on the IF output allowing for sufficiently sensitive RF detection.
The cavity based radar devices typically require a horn antenna up to several inches long and a cavity oscillator at least one or more cubic inches in size for operation at the 10 GHz or 24 GHz ISM bands (e.g., the X, and K bands). Both of these factors cause the system to have significant, weight and size, which is undesirable for a small hand-held application. Furthermore, the optimum Gunn diode bias point often requires substantial current draw, limiting the useful operating time for portable, battery-powered applications. Alternatively, the radar size must increase to accommodate larger batteries.
Another design approach to small sized radar devices uses planar or “patch” antenna arrays. These devices either use cavity stabilized Gunn oscillator/detectors or use traditional switching mixers where the LO signal switches the RF signal phase to the IF output dependent upon LO phase. The switching type of mixer typically shows 6 dB or more conversion loss, and must be a balanced configuration to cancel any AM noise of the local oscillator. Diodes used in conventional mixer-based systems act like switches that provide either an open circuit or a closed switch in a signal path. The LO signal drives the mixer diode(s) to turn the diode “on”, or low impedance, for about a half cycle and “off”, or high impedance, for the other half cycle.
The balanced or double or triple balanced switching diode mixer suffers from imperfect AM noise cancelation due to variations in manufacturing and remains sensitive to the AM noise of most oscillators. The down-converted local oscillator AM noise obscures the incoming RF signal, even while the local oscillator phase noise cancels due to the short time required for the round trip on the radar path or the path inside the mixer itself. Conventional (incoherent) receivers do not typically see the AM noise of the LO as the phase noise typically dominates the AM noise by tens of dB. Only in coherent reception (such as used for CW radar) does the phase noise of the LO cancel and allow the AM noise to dominate.
Additionally, the IF output of a switching diode mixer typically requires termination with a low noise IF amplifier with low input impedance, usually equal to 50 ohms. The noise voltage of that amplifier with 6 dB mixer loss is equivalent to twice that noise voltage measured at the antenna input. Diodes typically add another 0.5 to 1 dB to the input noise of the mixer above the conversion loss, further degrading the receive signal to noise ratio as seen at the antenna RF port. This type of radar does not typically deliver good long range performance compared with the Gunn and horn antenna alternatives without the addition of other components such as additional antennas or an RF preamplifier.
Other devices constructed using planar patch antenna arrays have used a Gunn-based cavity oscillator for the transmitter source and a detector diode for the receive mixer. These can provide reasonable AM noise from the Gunn source, but are limited in miniaturization by the size of the oscillator resonant cavity.
Components for radar systems and other applications overcoming the deficiencies of the prior art are desirable.